Veteran striker Ibrahimovic returned to the Rossoneri in January on a deal that runs until June with the option to extend his stay by a further 12 months.
Ibrahimovic, who won Serie A during his first stint with Milan, has scored three goals in six games across all competitions since re-joining and was in the starting line-up for Friday's (AEDT) Coppa Italia semi-final first leg against Juventus.
Massara hailed Ibrahimovic's quality and said the Swede will be the one to decide his future.
"It'll ultimately be up to him, how he feels and how long he wants to continue," Massaro said.
"He truly is a phenomenon, and we are very happy with his performance."
Milan is 10th in Serie A after 23 games, 10 points adrift of the top four but only two away from Hellas Verona in sixth.